In the last few years, the words "label" and "tag" have gotten a whole new meaning on social networks. del.icio.us, Gmail, flickr... you name it, you can probably tag it. Aside from the definite usefulness and convenience, tags are interesting to me because they are an example of an idea which broke the mold.
Try if you can (if you use Hotmail Windows Live Mail, it shouldn't be too hard) to remember what email was like without labels and tags (which are the same thing). We used these things called folders, and each email (oh yeah, we didn't have conversations, either) could only be in old folder at any time. The terminology is not surprising, because we borrowed that from how people organize real world letters on their desktop (I hope you're making the connection now). If you had a physical letter, then it could only go in one folder - unless you make copies, but then the copies don't reflect each other. If you highlighted one letter or made annotations, the other copy in another folder won't change to reflect that. Because this was the only way of treating communication, it was the model used for the first implementations of email.
Now, what I just said wasn't quite true. There are physical ways of giving an object several categories at once. They're what we call post-its. You can put different colored post-its in a book to note where, say, the author talks about the life of that time period versus the symbolism of socks. If they happen to be on the same page, no problem - you can tell the two apart because they're different colored. The only problem was that you couldn't tag a page with too many things - there's only so much page border for the post-its to stick out (not to mention that they only sell post-its in so many colors, although this can be mitigated by writing the theme on the post-it - hey, labels!).
When you think about it, tags are the logical extension of that. As mentioned, there is no limit to how many colors/labels you can have. This is only restricted by your ability to create strings (er, computer strings, that is, a series of characters). The adhesive on the post-it will also never die, and you can put as many labels on one thing as you want.
The digital tag can be thought of as a superset of folders. If you only put one tag on each object, then they act in the same way as folders. You can even put slashes (/) in the tags to simulate nested folders. And why not? Tags are more powerful than folders, and can keep the same feature (that is, it's backwards compatible). It seems to me that eventually the tag system will be moved offline to the desktop as well (if cloud computing doesn't completely eliminate the desktop market). I'm sure there are difficulties (the datastructures in the OS would have to be completely reorganized, for example), but I think the final result would be worth it.
On a side note: shortcuts on Windows, or links on Linux, allow sort of the same funcationality. One difference is that (for shortcuts and soft links) what is stored is not actually the file, but directions on how to find the file. It's like looking in a folder for one thing, then having something there telling you to look in another folder. Linux does offer hard links, which puts the actual file there (I'm dumbing this down; if you know better, good for you), but that's all just technical talk.
I really admire the person who thought to bring tags to the computer
it was the del.icio.us folks. Kudos), because they're seeing through the
limitations of the current system and doing something better (although
it turns out not to be "new" in this case). In my mind I associate it
with Newton and Leibniz seeing through functions and discovering
calculus, or Einstein cutting through the luminiferous
ether to arrive at
relativity. Of course, tags are not quite as history making as those
events, but there is something similar in the minds behind all of these
I tried to think of more instances where people have taken ordinary concepts, applied it to computers, then created a more powerful and more general version of it, but I didn't come up with anything (except some weak ones, like bookmarks). Can you find any examples?
"bookmarks" (more like book darts now)